In an electronic component unit, various types of electronic component including heat generating elements are included. Examples of heat generating elements are semiconductor switching elements, such as power field effect transistors (FETs), and shunt resistors. Power FETs may be used to control drive current in motors. In order to efficiently dissipate heat generated by the heat generating elements so as to be within their permitted temperature ranges, for example, a heat sink is provided. For example, in JP-H07-161925-A and JP-UM-3128955-B, a heat sink is employed for external dissipation of heat transferred from the heat generating elements to a circuit board.
In the electronic component unit of JP-H07-161925-A, a metal base board and a circuit board are separately attached to a heat dissipation case (corresponding to a heat sink). Heat generating elements are mounted to the metal base board via an insulating layer, and other electronic components are mounted to the circuit board. However, in JP-H07-161925-A, wiring leads for electrical connection between the metal base board and the circuit board are required, thereby increasing the number of constituent elements and the number of assembly processes.
In the electronic component unit of JP-UM-3128955-B, heat generating elements are mounted on a circuit board, and a tin layer is provided on the back face of the circuit board via a copper foil layer. A heat dissipation sheet (corresponding to a heat sink) is laminated on the tin layer, and fixed to the copper foil layer by melting the tin layer in a reflow oven. In JP-UM-3128955-B, the circuit board and the heat dissipation sheet should be electrically insulated. When an insulative member is simply interposed between the circuit board and the heat dissipation sheet, the number of constituent elements and the number of assembly processes will be increased.